Various types of electrical devices have been equipped with flat electrodes. Such devices include for example organic devices, either in the form of an organic light emitting diode or a solar cell, in which a stack of layer consisting of organic materials is arranged between device electrodes placed opposite each another. In these or other devices, the electrodes are produced by depositing an electrode material on a substrate surface. In this context, the substrate on which the flat electrode is deposited may be a device substrate on which the device is produced, as a layer stack, for example. But device layers that have been applied previously, organic layers, for example, or inorganic barrier layers, may be used as the substrate base on which the flat electrode is deposited subsequently.
A method for producing a flat electrode in the form of silver nanowires has been suggested in which a substrate is coated with a network of metal nanowires in the form of metal particles. In order to deposit the silver nanowires (AgNW), they are sprayed, printed or dipped. The silver nanowires that are currently offered on the market are produced in a wet chemical process. In order to ensure the shape of the wire, and later the solubility of the AgNWs in solvents such as water and alcohols, it was suggested that a stabiliser be added during the synthesis. Poly(vinyl)pyrrolidone (PVP) is often used in conjunction with AgNWs. The polymer stabiliser bonds to the surface of the wire during synthesis and remains in the form of a thin layer from 1 to 3 nm thick even after the wires have been cleaned. Since PVP is an electrical insulator, the contact resistance between the nanowires is accordingly very high, and the newly produced electrodes therefore have a high sheet resistance.
In order to lower sheet resistance, processes such as mechanical pressing, thermal, nanoplasmonic or ohmic annealing are used. In addition, AgNWs are combined in a conductive layer with other substances such as polymers, graphene or graphene oxide, carbon nanotubes, metal or oxide nanoparticles. One method that is often used is thermal annealing of the AgNW electrode. Depending on the dimension (diameter) of the AgNWs, temperatures from 120 to 250° C. were applied for periods from 15 to 120 minutes (Sachse et al., Organic Electronics, 14(1), 143-148, 2013; Sepulveda-Mora et al., Journal of Nanomaterials, 2012, 1-7; M. Song et al., 4177-41 84.). In this process, PVP melts and is partly destroyed, which leads to deformation of the wires (“spaghetti effect”), an enlarged contact area between the wires, and sometimes causes melting at the junctions points. This in turn results in a lower electrode sheet resistance. The thermal treatment of AgNW electrodes is costly in terms of both time and energy. It is difficult or even impossible to use on flexible polymer substrates, because many polymer films are destroyed or lose their flexibility and translucency (transmission) at the necessary or optimal processing temperatures. Lowering the temperature prolongs processing times, which is technologically disadvantageous.
Other methods known from the literature rely on the effects of heat: HIPL (High-Intensity Pulse Light) (Jiu et al., Nanoscale, 5, 11820-11828, 2013), surface plasmonic (Garnett et al., Nature Materials, 11(3), 241-92012, 2014) and ohmic nanowelding (Song et al., ACS Nano, 8(3), 2804-281 1, 2014) of AgNWs. The network is heated locally or over the entire surface by light or electricity, and these literature sources claim a particular, advantageous warming of the contact points for individual methods. HIPL, surface plasmonic and ohmic nanowelding take effect in a very short period of time, but they require the use of additional, complex equipment and devices.
The alternative method, using mechanical pressing to lower contact resistance, was demonstrated for glass and polymer substrates (Gaynor et al., Advanced Materials (Deerfeld Beach, Ha.), 25(29), 4006-1 3, 2013; Hu et. al, ACS Nano, 4(5), 2955-63, 2010; Tokuno et al., Nano Research, 4(12), 1215, 2011). In this context, processing usually took place at an elevated temperature (>80° C.) and a pressure of 25 MPa to 81 GPa. Mechanical pressing is expensive and better suited for polymer substrates because there is a risk that glass will break. Furthermore, the wires can become deformed, which impairs the electrode's ability to transmit (Gaynor et al., Advanced Materials (Deerfeld Beach, Fla.), 25(29), 4006-1 3, 2013; Hu et al., ACS Nano, 4(5), 2955-63, 2010).
Another way to lower the contact resistance is to create hybrid layers. In this way, highly conductive electrodes were created by applying PEDOT:PSS to AgNWs (Dong Yun Choi et al, Nanoscale, 5(3), 977-983, 2013). In this case too, an enlargement of the contact area between overlapping nanowires was observed. Electrodes that were produced at room temperature manifested similar properties to heated electrodes: 10.76 Ohm/sq with 84.3% transmission. One disadvantage of a PEDOT:PSS layer is its undesirable parasitic absorption. PEDOT:PSS is also acidic and can cause undesirable corrosion of nanowires.